Rubber compositions, including rubber compositions used for tire treads, are sometimes quantitatively reinforced with silica with only a minor amount of carbon black reinforcement.
The electrical conductivity of the rubber silica reinforced rubber composition is substantially reduced where very low levels of carbon black reinforcement are used. Such electrical conductivity reduction may be represented, for example, by an increase in the rubber composition's volume resistivity.
Conventionally, a coupling agent is used in conjunction with the silica to couple the silica to the elastomer(s) of the rubber composition. For liquid coupling agents, a carrier for the coupling agent, such as carbon, might be used to introduce it into the rubber composition where the coupling agent and the silica are subsequently combined in-situ in the rubber composition. In such case, the liquid coupler is pre-deposited on the carbon black prior to mixing it with the rubber composition.
The use of carbon black for such carrier and introduction of coupler and silica separately into the rubber composition is well known to those skilled in such art.
However, the use of relatively low levels of carbon black in the rubber reduces the electrical conductivity of the rubber composition which may be of concern for a tire tread where it is desired for the tire tread to dissipate static electricity build-up in the tire on a vehicle during use or operation of the vehicle over a road.
It is observed that a carbon black having a DBP value in a range of about 70 to about 120 cm.sup.3 /100 g with a corresponding BET value in a range of about 40 to about 159 m.sup.2 /g, namely N330 ASTM designated carbon black, is conventionally used as a carrier for liquid silane-based coupling agents for the silica, particularly where the coupling agent is a liquid bis(trialkoxysilylalkyl) polysulfide.
In one aspect of this invention, it is desired to reduce the volume resistivity of a rubber composition which is quantitatively reinforced with silica and only a minimal amount of carbon black, particularly where the rubber composition is to be used as a tire tread for contacting the ground.
Such volume electrical resistivity may suitably be determined by DIN 53682 or ASTM Method D257-92.